A day after the death of the seventh victim of a fiery refinery accident, the state of Washington widened its probe into the fatal event.

The Washington State Department of Labor & Industries has opened investigations into two contractors who serviced units tied to the April 2 accident at Tesoro Corp.'s Anacortes, Wash., refinery, the agency said in a statement Monday.

The contractors under scrutiny were hired by San Antonio-based Tesoro to unseal heat exchangers and to pressure wash the vessels once they were opened.

A heat exchanger transfers heat from one unit to another.

The explosion at Tesoro came after the exchangers had been cleaned, the state agency said. It described the inspections of the contractor as a routine step in its overall investigation.

The Washington state agency said asbestos is being removed from the blast area, but it could be another week before its inspectors can enter the site.

Asbestos used in refinery piping was scattered by the explosion and damaged piping continued to shed asbestos in the area. The agency said as the asbestos cleanup continues, the plan is to remove mangled scaffolding around the heat exchangers and build new ones so investigators could examine the damaged machinery.

?There may be no safe way to erect scaffolding around the heat exchangers because supports holding the machinery in place may have been extensively damaged in the explosion,? the agency statement said.

Now the plan is to dismantle the heat exchangers in phases so inspectors can examine them as they are taken apart.

The accident at San Antonio-based Tesoro's plant is the worst in the industry since the BP's Texas City disaster in March 2005 in which 15 workers were killed and 180 injured.

The U.S. Chemical Safety and Hazard Investigation Board also is investigating the accident. On April 5, the safety board's lead investigator said that three Tesoro workers were killed immediately by a fireball at the plant when a vapor cloud of hydrocarbons ignited. The employees were working about 50 feet from a unit used to make high-octane fuel.

Four other Tesoro employees suffered burns in the accident and were airlifted to a Seattle hospital, where they died.

Meanwhile, state inspectors continue to interview employees and managers and review documents related to operations of the facility.

Tesoro has said it will shut the refinery by the end of this month for an unspecified time to make repairs.